First off, Destro did extremely well for both Seasons 3 and 4. Hell, it got two massive nerfs in Season 4 and was still being brought to the vast majority of high keys, and it was still one of the best specs in the game on several bosses (for what little that’s worth in Season 4 since everything was easy).
Second off, I’m not saying “destro deserves to be uncompetitive,” either. You’re putting words into my mouth. In fact, I’d argue it isn’t even uncompetitive right now since it has very clear niches on Mythic Sennarth and Mythic Kurog Grimtotem, neither of which is a particularly easy boss, due to its still-excellent 2-target cleave with Havoc. All I’m saying is that Destro was never quickly nor adequately nerfed come Sepulcher or Season 4, which is factually correct considering that it was extremely dominant in both raid and M+.